pH Tolerance in Freshwater Bacterioplankton: Trait Variation of the Community as Measured by Leucine Incorporation

Abstract

b pH is an important factor determining bacterial community composition in soil and water. We have directly determined the community tolerance (trait variation) to pH in communities from 22 lakes and streams ranging in pH from 4 to 9 using a growth-based method not relying on distinguishing between individual populations. The pH in the water samples was altered to up to 16 pH values, covering in situ pH ؎ 2.5 U, and the tolerance was assessed by measuring bacterial growth (Leu incorporation) instantaneously after pH adjustment. The resulting unimodal response curves, reflecting community tolerance to pH, were well modeled with a double logistic equation (mean R 2 ‫؍‬ 0.97). The optimal pH for growth (pH opt ) among the bacterial communities was closely correlated with in situ pH, with a slope (0.89 ؎ 0.099) close to unity. The pH interval, in which growth was >90% of that at pH opt , was 1.1 to 3 pH units wide (mean 2.0 pH units). Tolerance response curves of communities originating from circum-neutral pH were symmetrical, whereas in high-pH (8.9) and especially in low-pH (<5.5) waters, asymmetric tolerance curves were found. In low-pH waters, decreasing pH was more detrimental for bacterial growth than increasing pH, with a tendency for the opposite for high-pH waters. A pH tolerance index, using the ratio of growth at only two pH values (pH 4 and 8), was closely related to pH opt (R 2 ‫؍‬ 0.83), allowing for easy determination of pH tolerance during rapid changes in pH. By measuring growth of a bacterium in a range of environmental conditions, such as temperature, pH, and salinity, the fundamental niche of the bacterium can be determined. The response to these varied conditions can also be seen as a descriptor of the tolerance of that strain to different environmental factors. In a similar way, the tolerance of a bacterial community in a habitat can be estimated by measuring the intrinsic growth of the community at a range of different environmental conditions. The response curves of a community can then be regarded as the trait distribution (community tolerance) of that community. In soil this approach has been commonly applied to study community tolerance to heavy metals (1-3), other toxicants (4, 5), temperature (6-8), and salinity (9, 10), but this method has been applied less often in aquatic habitats (11)(12)(13).pH has been shown to be a decisive environmental factor determining the bacterial community composition in both soil (14-16) and water (17-22), often being the most important one compared to factors such as temperature and moisture in soil (15) and temperature, water retention time, organic matter, and nutrient concentrations in freshwater systems (17,(19)(20)(21)(22). pH is also an environmental factor that can vary greatly in aquatic systems. Lake and stream waters can have pH values below 4 and above 9 even within small geographical areas (such as the area in southern Sweden in the present study). In highly productive lakes, surface pH can be as much as 2 U higher than in bottom waters, a var...

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